Method of producing rough polysilicon by the use of pulsed plasma chemical vapor deposition and products produced by the same

ABSTRACT

A method for depositing a rough polysilicon film on a substrate is disclosed. The method includes introducing the reactant gases argon and silane into a deposition chamber and enabling and disabling a plasma at various times during the deposition process.

This application is a Divisional of application Ser. No. 09/732,624,filed on Dec. 8, 2000 now U.S. Pat. No. 6,472,320 which is aContinuation of application Ser. No. 09/127,159, filed on Jul. 31, 1998now U.S. Pat. No. 6,214,726 which is a Divisional of application Ser.No. 08/762,544, filed on Dec. 9, 1996 which issued on Dec. 15, 1998 asU.S. Pat. No. 5,849,628.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the field of integrated circuitmanufacturing and, more particularly, to a method for forming a roughpolysilicon film.

2. Description of the Related Art

In semiconductor manufacturing, it is sometimes desirable to depositpolysilicon film on a substrate in such a way that rough polysiliconresults. In dynamic random access memory (DRAM) technologies, forinstance, it may be desirable to manufacture a memory cell capacitorusing rough polysilicon.

A capacitor having a given lateral area manufactured with a roughpolysilicon plate will have a higher capacitance than a capacitor ofhaving the same lateral area manufactured with a smooth plate. The roughpolysilicon capacitor has a larger effective dielectric surface area dueto the folding of the capacitor dielectric over the rough film. Whenused in a DRAM memory cell, the larger dielectric surface area increasesthe memory cell's capacitance and improves the cell's charge storagecharacteristics, thereby improving product performance.

Unfortunately, the use of traditional manufacturing methods to producerough polysilicon have been generally unsatisfactory because of thewidely varying roughness they produce. The instant process results in amore reliable rough polysilicon film that is more easily controlled.Moreover, because the instant process uses etchants and techniquescommon to the semiconductor industry, this improved process can bereadily integrated into modern day manufacturing flows.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, there isprovided a method for depositing a rough polysilicon film on asubstrate. The method includes the steps of introducing reactant gasinto a deposition chamber containing the substrate, and enabling anddisabling a plasma within the deposition chamber.

In accordance with another aspect of the present invention, there isprovided a method for manufacturing a capacitor having a plate formed ofrough polysilicon. The plate is formed by introducing reactant gas intoa deposition chamber containing the substrate and selectively andrepeatedly enabling and disabling a plasma to deposit the roughpolysilicon on the substrate. A dielectric layer is formed on the roughpolysilicon, and a conductive plate is formed on the dielectric layer.

In accordance with a further aspect of the present invention, there isprovided a capacitor that includes a polysilicon plate formed byintroducing reactant gases into a deposition chamber and selectively andrepeatedly enabling and disabling a plasma at various times during thedeposition process. A dielectric film is disposed on the polysiliconplate, and a top plate is disposed on the dielectric film.

In accordance with yet another aspect of the present invention, there isprovided a memory circuit that includes a plurality of memory cells.Each of the memory cells has a capacitor. Each capacitor has a plate ofrough polysilicon formed by introducing reactant gas into a depositionchamber and selectively and repeatedly enabling and disabling a plasmaat various times during the deposition process.

In accordance with a still further aspect of the present invention,there is provided a method for depositing a rough polysilicon film on asubstrate. The method includes the steps of: (a) placing a substrate ina deposition chamber; (b)introducing reactant gas into said depositionchamber; (c) creating a plasma; (d) disabling said plasma; (e) pumping aportion of reactive by-products out of said deposition chamber; and (f)repeating steps (c), (d), and (e) until a roughened polysilicon film hasbeen deposited on said substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparentupon reading the following detailed description and upon reference tothe drawings in which:

FIG. 1A is a cross-section of a bottom plate of a capacitor comprised ofrough polysilicon.

FIG. 1B shows the addition of a capacitor dielectric over the roughpolysilicon film.

FIG. 1C shows the addition of a top plate of a capacitor over thecapacitor dielectric.

FIG. 2 is a stylized representation of a polysilicon deposition chamber.

While the invention is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. However,it should be understood that the invention is not intended to be limitedto the particular forms disclosed. Rather, the invention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Turning now to the drawings, the formation of a capacitor, generallydesignated by a reference numeral 10, using a rough polysilicon is shownin cross-section in FIGS. 1A through 1C. In FIG. 1A, a rough polysiliconfilm 12 is formed over a substrate 14. The rough polysilicon film 12constitutes the bottom plate of the capacitor 10. In FIG. 1B, adielectric 16 is grown or deposited on top of the rough polysilicon film2. In FIG. 1C, a layer of conductive material 16 (e.g., polysilicon) isdeposited over the dielectric 16. The conductive material 18 constitutesthe top plate of the capacitor 10. In one embodiment, pulsed plasmachemical vapor deposition (CVD), using a mixture of silane gas (SiH₄)and argon (Ar), is used to deposit the rough polysilicon film 12 on theunderlying substrate 14.

A stylized representation of a deposition chamber 20, which can be usedto practice the disclosed process, is shown in FIG. 2. The substrate 14is shown positioned in the deposition chamber 20 atop a wafer chuck 22.A heating element 24 runs through the wafer chuck 22. The temperature ofthe heating element 24 may be varied by a temperature controller 26. AnRF power source 20 sends an oscillating voltage at a controlledfrequency to a plate 30 placed above the wafer chuck 22. The plate 30 isusually grounded to create a radio-frequency (RF) controlled plasma. Thevoltage that builds up between the plate 30 and the wafer chuck 22creates an electric field inside of the deposition chamber 20. When thishappens, gases that have been introduced into the chamber via a port 32and a port 34 can become ionized so that the reactants are depositedonto the substrate 14. This is commonly referred to as “striking” or“enabling” a plasma.

More than just two gases can be introduced into the deposition chamber20 at one time if the reaction chemistry so dictates, as one of ordinaryskill will realize. A suitable deposition chamber 20 for practicing thedisclosed embodiment is an Applied Materials™ model 5000D depositionchamber. This model is advantageous because it contains plasmageneration capabilities. Other more standard CVD chambers, such as theApplied Material™ Centura™ model 5200 or the AG Associates™ model 8000,may also be used if modified to generate a sufficient electric field toenable a plasma.

In the disclosed embodiment, the plasma is “pulsed,” meaning that theplasma is selectively enabled and disabled at various times duringdeposition. Pulsing of the plasma can be achieved by several means. Onesuch method involves connecting a pulse generator 36 to the RF powersource 28, as shown schematically in FIG. 2. The pulse generator 36sends a square wave to the RF power source 28. The square wave isconnected to the RF power source 28 in such a manner that the RF powersource 28 is enabled during the high portions of the square wave 28 andis disabled during the low portions of the square wave. When the RFpower source 28 is enabled, it sends an oscillating voltage to the plate30 as described above, thus facilitating formation of plasma in thedeposition chamber 20. When the RF power source 28 is disabled, theoscillating voltage is not sent to the plate 30. Thus, no plasma isformed during that period. It is also possible that the RF power source28 can be enabled and disabled by the use of a computer or by the use ofanother appropriate switch.

For sufficient production of a rough polysilicon film, the RF powersource 28, in this embodiment, is capable of producing a power in therange of 10 to 1,000 Watts and oscillates (when enabled) with afrequency of 400,000 to 15,000,000 Hertz (Hz), with 13,600,000 Hz beingtypical. The pulse generator 36 is capable of producing a square wavewith a frequency of 25 to 1,000 Hz. The duty cycle, i.e., the percentageof the time that the square wave is high, may vary between 10% and 90%.Assuming a six-liter volume for deposition chamber 20, the followingother deposition parameters are suitable for the production of a roughpolysilicon film:

deposition temperature of 300 to 600 degrees Centigrade; silane gas flowrate of 5 to 500 cubic centimeters per minute; argon gas flow rate of 10to 1000 cubic centimeters per minute; and deposition chamber pressure of0.1 to 100 Torr.

The pulsing of the plasma has distinct advantages. First, pulsing theplasma allows for stringent control of the plasma chemistry. During thedisabled portion of the plasma cycle, the reactive by-products arepumped out of the deposition chamber 20 by the pump 40, and the reactionis started anew on the next enabled cycle. If the plasma is not pulsed,the silane ions produced in the plasma will combine to form siliconwhich, when situated on the wafer surface, may produce a smooth film.

Another advantage of pulsing the plasma is that gas phase nucleation isgenerally not conducive to precise control, because nucleation size anddensity vary as a function of deposition time in an active plasma. Bykeeping the enabled time relatively short, these effects, which tend tocause the polysilicon film to grow in a random and unrepeatable fashion,are reduced. With a pulsed plasma, a constant desired concentration ofreactive silane can be maintained because during the disabled state, thesilane ions become neutral through recombination.

The disclosed process has other advantages as well. First, because themean free path of the reactive species is kept to a maximum bycurtailing the duration of the “enabled” portion, better polysiliconstep coverage can result. This makes the disclosed process beneficialfor depositing a rough polysilicon film on a three-dimensional surface,such as a contact hole, a trench, or a stack for a DRAM capacitor. Inaddition, rough polysilicon films using the disclosed technique can bedeposited at relatively low temperatures, such as 500 degreesCentigrade. Deposition at low temperatures is beneficial because thegrowth of the polysilicon film on the surface of the wafer will not bedictated by the thermal mobility of the silicon atoms which dissociatefrom the silane ions at the wafer surface. By the use of lowerdeposition temperatures, a rough polysilicon film can be achieved thatis largely independent of temperature. By contrast, conventionalnon-pulsed CVD rough polysilicon films cannot typically be deposited atsuch low temperatures, because the resulting film will be amorphous and,therefore, smooth.

The use of the disclosed method produces a film in which the vertical“peak-to-valley” roughness of the polysilicon film is approximately 500Angstroms, and the lateral “peak-to-valley” roughness of the polysiliconfilm is approximately 500 Angstroms. However, the resulting film is afunction of the various processing conditions that are discussed herein,and the process is susceptible to optimization before a film of asuitable rugosity is realized for a given application.

What is claimed is:
 1. A capacitor comprising: a first plate comprisedof a rough polysilicon film; a dielectric film disposed over the firstplate, wherein the dielectric film has a vertical peak-to-valleyroughness of approximately 500 Angstroms; and a second plate comprisedof a conductive film disposed over the dielectric film.
 2. Thecapacitor, as set forth in claim 1, wherein the dielectric film conformsto the rough polysilicon film to provide a roughened surface to thesecond plate.
 3. The capacitor, as set forth in claim 1, wherein theconductive film comprises polysilicon.
 4. The capacitor, as set forthclaim 2, wherein the conductive film conforms to the roughened surfaceof the dielectric film to form a roughened surface on the second plate.